These measurements are necessary, for example, for the adjustment of implants, such as intraocular lenses after cataract surgery.
However, due to the increasing complexity and desired individualization of the implants, measurement data of more than one depth profile (A-scan) of the eye, particularly of several laterally offset A-scans, are required.
Other ophthalmological devices serve primarily for producing and evaluating two-dimensional images, sectional images, and volume scans of various areas of the eye with regard to the visual impression, sizes and distances of certain eye structures. Solutions thereto are known from prior art which apply optical scan systems.
A first group thereto are tomography systems, which, e.g., are based on the so-called OCT (optical coherence tomography) method, whereby coherent light is employed on reflective and scattering samples with the help of a interferometer for distance measurements and imaging. Through depth scans, the optical coherence tomography on the human eye delivers measurable signal responses due to the changes in the index of refraction at optical boundaries and due to volume scattering.
For example, the basic principle of the OCT method described in U.S. Pat. No. 5,321,501 is based on white-light interferometry and compares the duration of a signal by means of an interferometer (most often a Michelson interferometer). Thereby, the arm with known optical path length (=reference arm) is used as reference to the measurement arm, in which the sample is located. The interference of the signals from both arms results in a pattern with which the scattering amplitude, in dependence of the optical delays between the arms, can be determined, resulting in a depth-dependent scattering profile which, analogous to ultrasound technology, is called an A-scan. Quick variations of the optical delays between measurement and reference arm can be realized, e.g., by means of fiber links (EP 1 337 803 A1) or so-called rapid-scanning optical delays (RSOD) (U.S. Pat. No. 6,654,127 B2). In the multidimensional raster scan method, the beam is then transversally guided in one or two directions, allowing for a two-dimensional B-scan or a three-dimensional volume tomogram to be recorded. If the length of the reference arm is kept constant, a two-dimensional C-scan can be recorded through lateral scanning of the measurement beam in two directions.
The solution described in US 2007/0291277 A1 also refers to a device based on the OCT method. Contrary to the previously described solution, a Mach-Zehnder interferometer with a fiber reference path is used herein. Imaging of the optical coherence scanning device is said to be improved through different auxiliary equipment and/or functions, such as a fundus imaging device, an iris viewer, a motorized chin rest, and internal control of the instrument alignment.
A second group consists of confocal scanner-based ophthalmoscopes, particularly confocal scanning laser ophthalmoscopes (cSLO) which, in addition to the OCT-based tomography systems, also represent known and important tools for diagnosis and therapy in ophthalmology (U.S. Pat. No. 6,769,769 B2).
Confocal scanners can achieve a three-dimensional spatial resolution through limiting the depth of a spatially-adjustable focus by means of spatial filtering and, compared to OCT, do not rely on the utilization of interference effects.                Solutions for laser scanning ophthalmoscopes are described, for example, in U.S. Pat. No. 7,284,859 A and U.S. Pat. No. 7,374,287 B2.        
Aside from fluorescence or indocyanine green angiography, laser scanning ophthalmoscopes can also be used for detecting different retinal diseases from the recorded fundus autofluorescence images (FAF images). Through excitation with light of a suitable wavelength and the appropriate filters, different retinal diseases become detectable. Changes of the topographic FAF intensity distribution appear with various retinal pathologies, such as age-related macular degeneration (AMD), macular edema, and genetically determined retinopathies. As with OTC, a glaucoma diagnosis is possible through the measuring of form and size of the optic nerve head. With this method, scattering profiles are also obtained from the response signal. Since the backscatter at the optical boundaries is particularly high due to the refractive indices, the determination of the optical path lengths between the boundaries is therefore also possible (WO 2008/151821A1).
In order to obtain the images in the form of two-dimensional images, sectional images, and volume scans required for the diagnosis, scans, in addition to A-scans (individual depth profiles), are required transversally in a first (B-scans) and a second direction (volume scans). Thereby it is very important to record those scans very quickly since the available attention span of a patient (with barely 2 sec.) is very limited.
Therefore, very quick deflection systems for the measurement beams must be deployed in such imaging systems.
At the same time, said deflection systems must be able to render a predetermined scan pattern very accurately, linearly, and very reproducibly, ensuring that the emerging sectional images and volume scans exhibit no distortions which would make the evaluation of the structures needlessly difficult. This is a contradiction with conventional galvanometer scanners since the resonant variations may be able to scan very quickly but are very often limited to sine-like scan patterns. However, other patterns can only be realized with slower and more elaborate non-resonant galvanometer scanners.
Said great challenges to the deflection equipment regarding speed, control accuracy, and linearity are, e.g., further enhanced when so-called tracking systems, already widely used in ophthalmology, are used which detect, register and/or actively compensate the eye movements during the course of the measurements. Such tracking systems are described, for example, in U.S. Pat. No. 6,726,325 B2; US 2006/228011 A1; U.S. Pat. No. 6,325,512 B1; and U.S. Pat. No. 7,365,856 B2.
Alternatively to active tracking, tomogram distortions due to interfering eye movements can be corrected through appropriate referencing with quick, more stable OCT scans in certain directions (WO 2006/077107 A1).
In order to meet these requirements, stably rotating polygon mirrors for realizing close to linear sawtooth or triangular scans, or galvanometer mirrors within closed control loops are used, according to prior art. Polygon mirrors are able to scan very quickly and stably but are limited to a defined deflection pattern in a defined direction. Additionally, they are very loud and expensive.
By contrast, galvanometer mirrors are able to realize different scan patterns but also require great electronic control efforts (U.S. Pat. No. 6,956,491 B2 and U.S. Pat. No. 6,433,449 B1) in order to reproduce a predetermined deflection pattern with accuracy, linearity, and reproducibility acceptable for imaging.
Therefore, elaborate combinations of both deflection systems are also employed as optical scan unit in ophthalmological devices, whereby a quickly rotating polygon mirror produces a fixed deflection pattern in a deflection direction and a galvanometer mirror realizes a deviating and, within certain limits, flexible deflection pattern in a second, slower deflection direction (U.S. Pat. No. 7,374,287 B2 and U.S. Pat. No. 6,810,140 B2).
Further known optical scan units are positionally adjustable, diffractive or refractive optical elements, such as rotating prisms (S. Han et al., Journal of Biomedical Optics, Vol. 13, 020505-1 (2008), or electrostatically deflected miniature mirrors (MEMS, US 2008/0186501 A1), acousto-optical (AOM) and electro-optical modulators (EOM; U.S. Pat. No. 6,404,531 B1). In principle, deformable mirrors and liquid crystal modulators for beam deflection are also suitable but in general slow by comparison.
The solutions for deflection systems for scanning measurement value logging, which are known in accordance with prior art and described herein, have the disadvantage of being very elaborate and expensive.
The example of a system for measurement beam deflection for an OCT system, according to prior art (US 2007/0291277 A1) is described in FIGS. 1a and 1b. Thereby, the OCT system, shown in FIG. 1a as functional schematic, consists of an illumination unit, an interferometer, a signal detection, and a central control unit with output unit. For the realization of B- and C-scans as well as volume scans, the measurement arm of the interferometer contains a deflection and/or scan unit, which exhibits connections to the central control unit.
The central control unit activates a pattern generation for one or even both scan directions, which transmits the patterns to be realized to the additionally existing control of the optical scan unit. In order to ensure that the produced scan pattern corresponds as close as possible to the pattern to be mapped, the general arrangement also exhibits a control circuit or control loop with at least one position sensor, which supplies the feedback signal. While the control sets the pattern to be generated as nominal value, the actually mapped pattern is determined by the position sensor. In the control circuit, the differences between nominal and actual values are determined and utilized by the control unit of the optical scan unit for minimizing the amount [nominal—actual].
Thereto, FIG. 1b shows in a diagram a predetermined nominal pattern (dotted line) as well as an effectively mapped actual pattern (solid line). Due to the limited capabilities of the scan unit, the deviations are caused by a predetermined, particularly quickly changing pattern and can be minimized with great effort but never entirely avoided. Depending on the settings of the control circuit or control loop, an incomplete following (“creeping approximation”) of the actual pattern with regard to the intended pattern or, contrarily, overcompensation can occur (“overshoot” as shown in FIG. 1b). The properties of an optical scan unit including control circuit can be described through a transfer function between nominal and actual patterns, which can be measured and their properties taken into consideration for the pattern generation.
In the central control unit, respective signals are reconstructed from the acquired measurement values and images generated and, if applicable, biometric data calculated.